Fig. 22.7 Inflation-deflationpressure-volumecurve.Thedirectionofinspirationandexhalationisshownbythearrows.Thedifferencebetweentheinflationanddeflationpressure-volumecurvesisduetothevariationinsurfacetensionwithchangesinlungvolume.Notetheslopeofthelinejoiningpointsofnoairflow.Thisslopeislesssteepthantheslopefromthedeflationpressure-volumecurveatthesamelungvolume.
tidal volume, and more surfactant material is incorporated into the air-liquid interface. Therefore the lung is more compliant.
Sighing and yawning increase dynamic compliance by increasing tidal volume and restoring the normal surfactant layer. Both of these respiratory activities are important for D maintaining normal lung compliance. In contrast to the 1000 lung, the dynamic compliance of the chest wall is not significantly different from its static compliance.
EF Insp. Exp. B
Work of Breathing
Breathing requires the use of respiratory muscles (diaphragm, intercostals, etc.), which expends energy. Work is required to overcome the inherent mechanical properties of the lung (i.e., elastic and flow-resistive forces) and to move both the lungs and the chest wall. This work is known as the work of breathing. Changes in the mechanical properties of the lung or chest wall (or both) in the presence of disease result in an increase in the work of breathing. Respiratory muscles can perform increased work over long periods. However, the body. In the respiratory system, the work of breathing is calculated by multiplying the change in volume by the pressure exerted across the respiratory system:
Work of breathing (W) = Pressure (P) DC like other skeletal muscles they can fatigue, and respiratory failure may ensue. Respiratory muscle fatigue is the most common cause of respiratory failure, a process in which gas exchange is inadequate to meet the metabolic needs of
Although methods are not available to measure the total amount of work involved in breathing, one can estimate the mechanical work by measuring the volume and pressure changes during a respiratory cycle. Analysis of pressure-volume curves can be used to illustrate these points.
Fig.
22.8A represents a respiratory cycle of a normal lung.
The static inflation-deflation curve is represented by line ABC. The total mechanical workload is represented by the trapezoidal area OAECD.
In restrictive lung diseases, such as pulmonary fibrosis, lung compliance is decreased and the pressure-volume curve is shifted to the right. This results in a significant increase in the work of breathing (see
Fig. 22.8B ), as indicated by the increase in the trapezoidal area of OAECD. In obstructive lung diseases, such as asthma during an exacerbation or chronic bronchitis, airway resistance is elevated (see
Fig.
22.8C ) and greater negative pleural pressure is needed to maintain normal inspiratory flow rates. In addition to the increase in total inspiratory work (OAECD), individuals with obstructive lung disease have an increase in positive pleural pressure during exhalation because of the increase in resistance and the increased expiratory workload, which is visualized as area DFO. The stored elastic energy, represented by area ABCF of